Sludge stacking speeds sewage treatment

A new way of stacking sewage sludge has the potential to halve drying times for the waste, increasing the capacity of expensive infrastructure and ultimately saving water treatment utilities millions of dollars.

The University of Melbourne has been working with Melbourne Water to adapt the dry stacking technique, commonly used in the minerals industry. Professor Peter Scales has been leading the project.

Dry stacking in the minerals industry involves a chemically induced thickening process that changes the rheology of the mineral sludge and allows it to be stacked an angle so that water drains off.

However, thickening is not possible in the wastewater industry. So to create a similar effect, modifications to the drying pans and to the way biosolids are stacked in the pans are being trialled at Melbourne Water’s Western Treatment Plant.

Professor Scales says the solids are added to the drying pan in layers, rather than being pumped in en masse. This allows sun and wind evaporation to begin drying each layer before the next is added. This technique has almost halved drying time and allows more sludge to be stacked into the same drying bed. Rainfall also becomes less of an issue for the drying process, because it is able to run off the stack.

The increased capacity of existing drying beds will defer the need for multimillion-dollar investment in new wastewater treatment infrastructure or major upgrades for Melbourne Water.

Professor Scales says the management of sludge is a global issue, and drying before disposal is often expensive. Dry stacking uses low-cost solar energy, and has the potential to be adapted anywhere that evaporation exceeds rainfall — which includes large parts of Australia.